The present invention relates to a method of producing, in gaseous form and under high pressure, at least one fluid chosen from oxygen, argon and nitrogen, in which method air is distilled, the said fluid is brought in the liquid state to the high pressure, it is vaporized and warmed at this high pressure in the heat exchange line of the installation.
In the present specification, “high pressure” means a pressure greater than 10 bar in the case of oxygen, argon and nitrogen, and “blower” means a compressor having a single compression stage. Furthermore, the pressures in question are absolute pressures.
When producing oxygen, these methods, called “pumped” methods, have the advantage of dispensing with the oxygen compressor, which is a costly machine, poses serious reliability problems and has high maintenance costs.
EP-A-0 504 029 describes a method in which all the air is compressed to a high pressure in a blower, a portion of the high-pressure air is expanded in a Claude turbine (that is a Claude turbine which discharges into the medium-pressure column) and the rest of the air exchanges heat with the liquid oxygen in the process of vaporizing in the exchange line.
In this type of unit, it is desirable to have a means of preventing the inlet of the turbine becoming too cold, for example in the event of change of operation.
FR-A-2 688 052 describes a method in which:                air in the process of cooling in the heat exchange line is extracted from the latter at an intermediate temperature close to the vaporization temperature of the said fluid, or to its pseudo-vaporization temperature if the high pressure is supercritical;        this air is compressed in a blower;        it is reintroduced into the heat exchange line and at least one expansion of a cycle gas is effected in a turbine.        
EP-A-0 644 388 describes a method in which a portion of the air is compressed to the medium pressure and sent into the medium-pressure column of a double column while the rest of the air is supercharged at ambient temperature. A portion of the supercharged air is then compressed in a cold supercharger.
During the start-up of the units according to EP-A-0 644 388 and FR-A-2 688 052, the air extracted from the heat exchange line is at the inlet of the blower at ambient temperature due to the fact that there are very few cold gases that warm up in the exchange line. Following compression, it returns to a temperature that may be as high as 120° C., compared with the temperature of approximately −120° C. when the unit is in stable operation. This may damage the exchange line, which is not designed to withstand such high temperatures.